1. Field of the Invention
This invention concerns downflow filters, and more particularly concerns methods of backflushing multiple filter media layers in downflow filters to segregate the layers and enhance effectiveness of the filter.
2. General Discussion of the Background of the Invention
Downflow filters originally employed a single filter bed comprised of sand. Use of a single layer was a drawback, however, because sand consists of a variety of different grain sizes. The grain size distribution resulted in a stratification of granules during backwash. More specifically, during the backwashing step the filter beds expanded and the lighter, smaller particles tended to rise to the top of the bed and the heavier, larger materials tended to gravitate to the bottom. The single layer of sand thereby tended to stratify with the finest filter particles in the top of the filter and the coarsest filter particles in the bottom. This stratification created a condition in which the finest particles on top were subject to "surface plugging" which rapidly decreased the effectiveness of the filter.
This problem was overcome in the prior art by the provision of dual media filters. These filters typically consist of a top layer of anthracite and a bottom layer of sand, the anthracite being lighter and larger than the sand, and the sand consisting generally of the same size particles as used in the mono-bed filters described above. The top anthracite layer presents a more open and porous surface to the influent, thereby allowing coarser particles to penetrate the surface. After backflushing, the smaller, heavier sand particles tend to remain at the bottom of the filter because their specific gravity is greater than that of anthracite.
In spite of the advantages of the multi-media filters, they still present some serious problems. For example, backflushing tends to intermingle the particles of the respective layers of the filter. This intermingling is brought about by the fact that a layer of caked contaminants forms over the top portion of each of the respective beds. During backflushing, pressure builds up below the layer of contaminants and the caked contaminants initially break in a few places instead of breaking uniformly across the layer of the bed. As these initial breaks in the caked layer occur, particles of the medium underneath the caked layer jet up into the media above.
Another cause of intermingling among the layers is that lateral distributor pipes are usually employed at the bottom of the filter. These lateral distributors are comprised of a central header running across a diameter of the tank adjacent the bottom of the tank, with perforated secondary distributor pipes transverse to the central header. Backwash fluid under pressure is introduced into the central header and thence into the lateral distributors whence it moves through the perforations in the lateral distributors to fluidize some of the filter layers. These lateral distributors, however, tend to unevenly distribute fluid under pressure into the beds during backflushing, thereby creating a churning effect and very effectively intermingling the layers to defeat the purpose of backwashing.
The churning effect caused by lateral distributors can be slightly diminished by using a plurality of nozzles across a horizontal cross section of the filter tank. Even then, however, the nozzles tend to develop local areas of high velocity that move particles between layers.
Since it is critical to the proper performance of the downflow filter that the distinct layers of media remain well stratified, the problems of the prior art have created a serious and expensive problem for users of downflow filters. It has become the accepted practice in the field to remove the filter media from the filter container and replace it with new, stratified layers. This is a costly and time consuming procedure that may be necessary as frequently as two to three times a year.
It is accordingly an object of this invention to provide a method of backwashing multiple media downflow filters to restratify the media without removing them from the vessel.
It is another object of the invention to introduce backwash fluid under pressure into the filter media without creating areas of local high velocity within the media.